Currently, non-mechanical (i.e., optical) mice and pointing devices are commercially available. One example is disclosed in Japanese Patent No. 3771081. In this technology, an operation surface is illuminated with light at a low incident angle, specifically, 70° to 85°, and the microscopic textures of the surface are detected based on their highlights and shades. In general, an LED (light emitting diode) is often used for the light; a laser diode, particularly, a VCSEL (vertical cavity surface emitting laser), may also be used to improve the ability to recognize the microscopic textures.
Japanese Unexamined Patent Application Publication No. 2005-50349 discloses a pointing device that includes an optical motion-detecting circuit. This optical motion-detecting circuit determines the amount of movement by illuminating a surface with infrared light and determining a correlation between images based on light reflected from the surface. This motion detecting circuit (included in the pointing device) detects motion in a predetermined direction by correlating a reference frame, which is a digitized output of reflection light obtained by a light detector, and a sample frame, which is the next digitized output obtained after the reference frame.
In addition, for example, a motion detecting device disclosed in Japanese Unexamined Patent Application Publication No. 2004-246921 has been incorporated into a commercially available pointing device. This device includes a navigation sensor and a light source, such as a laser diode, for producing coherent light. The sensor detects reflection light resulting from light emitted from the light source to determine the amount of movement of a pointing device (e.g., a mouse).
The pointing device disclosed in Japanese Unexamined Patent Application Publication Nos. 2005-50349 and 2004-246921, however, may not operate accurately or may malfunction when used on a glass surface. This is because a glass surface is generally very smooth and the pointing device disclosed in these publications cannot detect microscopic textures, which are necessary for the pointing device to successfully perform detection. In addition, the surface roughness of the glass surface is about several nanometers, which is very small compared to the wavelength of incident light and is even very small compared to one pixel of the light detector, which also makes it difficult to detect the microscopic textures.
The signal intensity of the aforementioned reflection light will be briefly described next. In general, in order to detect the intensity of light reflected from a plate surface having a certain surface roughness as a light/dark pattern, it is required that the size of the light/dark pattern be larger than the light wavelength and be substantially the same as the size of one pixel used for detection.
Additionally, since the surface roughness of glass is about several nanometers, which is very small, it is difficult to perform high-sensitivity detection of the amount of change in the intensity of reflection light from such a surface.
With a mirror, the intensity of reflection light is equal to the intensity of incident light. With glass or the like, however, the intensity of reflection light varies depending on the reflection coefficient of its surface (the reflection coefficient depends on the polarization direction and incident angle of light). The average intensity IR of the reflection light is given by:IR=R·IIN  (Expression 1)
where R indicates the reflection coefficient and IIN indicates the intensity of the incident light.
In general, the average intensity Id of speckle light emitted from the surface is given by:Id≈(4πσ/λ)2·IR  (Expression 2)
There are demands for a pointing device, such as an optical mouse, that can be used on a smooth plate, such as a glass plate.